4,4’,4 "-Tris(diphenylmethylene) amine (N(p-DPM)(3)) may be regarded as a prototype of intramolecularly spin-frustrated organic systems. In the present work the monoanion of N(p-DPM)(3) was obtained by thermal electron attachment and studied by ESR spectroscopy to find that the ground state is a spin quartet with the fine-structure parameters of D = 0.123 cm(-1) and E = 0.005 cm(-1) and an isotropic g factor of 2.003. These results are explained consistently if the excess electron occupies an in-plane n non-bonding MO predominantly localized at the divalent carbon sites and if the molecular structure of N(p-DPM)(3)(.-) is of non-C-3 symmetry which indicates the quantum nature of the spin alignment in the spin-frustrated finite system. The temperature dependence of the ESR signal indicates that the lowest excited doublet state of the anion is located at least 300 cm(-1) above the quartet ground state.